The present invention relates to a driving force distribution control system for a four wheel drive vehicle, and more specifically to a system for controlling a transfer clutch or the like for distributing the driving torque of an engine between front and rear axles.
Japanese patent provisional publication No. 61-157437 discloses one conventional example. A driving force distribution control system of this example is designed to vary the driving force distribution toward a state of four wheel drive by increasing an engagement force of a transfer clutch in accordance with an increased difference between sensed speeds of the front and rear wheels, in order to prevent slippage of the driving wheels.
However, this conventional control system is unable to respond to a change in the friction coefficient of a road surface, in a quick and adequate manner because the characteristic of the driving force distribution control is fixed. If, for example, the characteristic of the driving force distribution control is determined so as to suit a dry paved road (that is, the share of the driving torque transmitted to the front wheels is held relatively small), then the tendency to spin is increased on a road of a low friction coefficient due to ice or snow, so that the four wheel drive system cannot provide satisfactory driving stability and the ability of escaping from adverse road conditions. If, on the other hand, the control characteristic is determined so as to attain a sufficient stability of driving on a slippery road surface (that is, the share of the driving torque distributed to the front wheels is relatively high), then the driving torque transmitted to the front wheels becomes too much on a high friction coefficient road surface, and the tendency to drift-out is increased.
It is possible to meet these problems by employing a driver's manual selector switch for changing the control characteristic. However, such a manual switch is troublesome for average drivers, and does not respond to abrupt changes in road conditions.